Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 338

The main objective of an anti-seismic design standards is to ensure three things:
the protection of human life during an earthquake, the operational continuity of
constructions important for emergency services, and the limitation of structural
damage. A major obstacle to this aim lies in the randomness of seismic action, the
statistical characteristics of which are difficult to determine in countries with low or
medium seismicity. Furthermore, earthquake protection has a cost, and some
countries have low disposable resources.
In the case of Eurocode 8 (Part 1), these general objectives are expressed by the
following mandatory prescriptions:
- Non-collapse prescription : structures must be designed and built to withstand -
without any local or global collapse - an earthquake, the aggressive level of which
corresponds to an overlapping probability over 50 years determined by the National
Authorities (10% in principle, which corresponds to a 475-year-return period). This
is the reference seismic action. After the event, the structure must preserve a notable
residual strength capacity (especially with regard to permanent loads), and should be
able to withstand a replica without collapsing.
- Damage limitation prescription : the structure should be designed and built so
that, under the effect of an earthquake more important than the previous one, the
structure will not suffer damage the repairing cost of which would be higher than the
cost of the structure itself, or which would limit its normal use. The earthquake level
sought corresponds to an overlapping probability over 10 years set by the national
authorities (usually 10%, i.e. a 95 year-return period). In order to make the
designer's task easier, such seismic action can be derived from the reference seismic
action by affinity.
- The specific protection assigned to certain types of constructions that are
important for public safety is ensured by means of a J I multiplying coefficient
(“ coefficient of importance ”) which is applied to the seismic action. This amounts to
increasing the return period (within the limits of the values considered for the
coefficient) or to decreasing the overlapping probability over a given period of time.
In order to make such differentiation possible, constructions are classified into four
importance classes, and a J I value is assigned to each class. Class IV corresponds to
construction works vital for public safety, whereas class II corresponds to standard
buildings.
9.3.3. Verification method
9.3.3.1. General principles
The Eurocode system is based on a series of common principles that apply to the
use of structural materials, either in standard operation situations or in particular